Most oil and gas wells are drilled by a drill string made up of drill pipe. At various depths and at the total depth, the operator will remove the drill pipe, then run in a string of casing. The casing lines the well bore and is cemented in place. In another technique, casing is employed as the drill string and when reaching a desired depth, it is cemented in place.
When running casing into a well either for casing the well or for casing drilling the well, normally a powered spider is employed at the drill rig floor. The powered spider is a device that encircles the hole in the rig floor. It has segments or slips that will slide down to a gripping position gripping the casing string suspended in the well bore. Fluid power, typically hydraulic, is employed to move the slips back to an upper position to allow the casing to be lowered into the well bore.
The spider will support the casing string while a new joint of casing is being made up to it. The new joint of casing will be hoisted by a casing lifting mechanism above the casing string suspended in the well. In one technique, the casing lifting mechanism comprises a casing gripper mounted to a top drive. The top drive runs up and down the derrick and also is capable of rotating a drill string or casing string. The casing gripper has gripping members that can be moved to a gripping position wherein they will engage a wall of the casing to support the casing. The grippers may engage either the inner diameter or the outer diameter of the casing. Typically the casing gripper is actionable by fluid power, such as hydraulic fluid, to move the grippers to their released position.
In another technique, rather than a casing gripper mounted to a top drive, the operator will employ a casing elevator, which is suspended by the blocks or a top drive in the derrick. The casing elevator is capable of gripping a string of casing and supporting the weight. The casing elevator is typically moved from the gripping position to the released position by fluid power.
After the operator makes up the new joint of casing with the casing string suspended by the spider, he will lift the entire casing string slightly, then release the spider to lower the casing string further into the well. When the upper end of the uppermost joint of the casing string nears the spider, the operator again engages the spider to support the casing string. The operator releases the casing lifting mechanism and repeats the process.
There are thus at least two valves that are controlled by personnel on the rig floor, one being to release the spider and the other being to release the casing gripping mechanism. If an operator accidentally moves the spider valve while the casing lifting mechanism is open, it is possible that the casing string could fall into the well bore. Normally the fluid release mechanism for the casing lifting mechanism is not sufficiently strong to release the casing lifting mechanism unless the weight of a joint of casing has been removed from it. However, many spiders have release mechanisms that will release a casing string while supporting it if the casing string weight is not very much. For example, that might occur when only a few joints of casing make up the casing string. It might also occur with a long casing string when this string is being run into a highly deviated well such as a horizontal well. It could also occur with under balanced drilling. There are some proposed solutions but improvements are desired.